Condensed Matter

A nonmonotonic dependence of the critical Josephson supercurrent on the injection current through a normal metal/ferromagnet weak link from a single domain ferromagnetic strip has been observed experimentally in nanofabricated planar crosslike S-N/F-S Josephson structures. This behavior is explained by 0-pi and pi-0 transitions, which can be caused by the suppression and Zeeman splitting of the induced superconductivity due to interaction between N and F layers, and the injection of spin-polarized current into the weak link. A model considering both effects has been developed. It shows the qualitative agreement between the experimental results and the theoretical model in terms of spectral supercurrent-carrying density of states of S-N/F-S structure and the spin-dependent double-step nonequilibrium quasiparticle distribution.

Unconventional superconductivity and magnetism are intertwined on a microscopic level in a wide class of materials, including high- T c cuprates, iron pnictides, and heavy-fermion compounds. A new approach to this most fundamental and hotly debated subject focuses on the role of interactions between superconducting electrons and bosonic fluctuations at the interface between adjacent layers in heterostructures. A recent state-of-the-art molecular-beam-epitaxy technique has enabled us to fabricate superlattices consisting of different heavy-fermion compounds with atomic thickness. These Kondo superlattices provide a unique opportunity to study the mutual interaction between unconventional superconductivity and magnetic order through the atomic interface. Here, we design and fabricate hybrid Kondo superlattices consisting of alternating layers of superconducting CeCoIn 5 with d -wave pairing symmetry and nonmagnetic metal YbCoIn 5 or antiferromagnetic heavy fermion metals, such as CeRhIn 5 and CeIn 3 . In these Kondo superlattices, superconducting heavy electrons are confined within the two-dimensional CeCoIn 5 block layers and interact with the neighboring nonmagnetic or magnetic layers through the interface. In CeCoIn 5 /YbCoIn 5 superlattices, the superconductivity is strongly influenced by the local inversion symmetry breaking at the interface. In CeCoIn 5 /CeRhIn 5 and CeCoIn 5 /CeIn 3 superlattices, the superconducting and antiferromagnetic states coexist in spatially separated layers, but their mutual coupling via the interface significantly modifies the superconducting and magnetic properties. The fabrication of a wide variety of hybrid superlattices paves a new way to study the relationship between unconventional superconductivity and magnetism in strongly correlated materials.

The superconducting state starts to collapse when the externally applied magnetic field exceeds the Meissner-Ochsenfeld critical field, Bc,MO, which in type-I superconductors is the thermodynamic critical field, while in type-II superconductors this field is the lower critical field. Here we show that both critical fields can be described by the universal equation of B c,MO = μ 0 n μ B ln(1+2 0.5 κ ), where μ 0 is the magnetic permeability of free space, n is the Cooper pairs density, and μ B is the Bohr magneton, and κ is the Ginzburg-Landau parameter. As a result, the Meissner-Ochsenfeld field can be defined as the field at which each Cooper pair exhibits the diamagnetic moment of one Bohr magneton with a multiplicative pre-factor of ln(1+2 0.5 κ ). In the two-dimensional case this implies that the Cooper pair center of mass is spatially confined within a ring with inner radius ξ and outer radius of ξ +2 0.5 λ , where ξ is the coherence length and λ is the London penetration depth. This means that the superconducting transition is associated not only with the charge carrier pairing, but that the pairs exhibit a new topological state with genus 1.

Electron energy loss near edge spectra (ELNES) has been calculated for two different Fe based superconducting material FeSe and FeTe using density functional theory (DFT). Fe K-edge absorption spectra consist of several features, origins of which are thoroughly described in light of partial density of states of constituent atoms. Here we have included "core-hole effect" and found drastic change in absorption spectra. Our results including core hole effect matches very well with experimental X-ray Absorption Near Edge Structure (XANES) results available in literature.

Based on first-principles calculations including a Coulomb repulsion term, we identify trends in the electronic reconstruction of A NiO 2 /SrTiO 3 (001) ( A= Pr, La) and A CuO 2 /SrTiO 3 (001) ( A= Ca, Sr). Common to all cases is the emergence of a quasi-two-dimensional electron gas (q2DEG) in SrTiO 3 (001), albeit the higher polarity mismatch at the interface of nickelates vs. cuprates to the nonpolar SrTiO 3 (001) substrate ( 3+/0 vs. 2+/0 ) results in an enhanced q2DEG carrier density. The simulations reveal a significant dependence of the interfacial Ti 3 d xy band bending on the rare-earth ion in the nickelate films, being 20 - 30% larger for PrNiO 2 and NdNiO 2 than for LaNiO 2 . Contrary to expectations from the formal polarity mismatch, the electrostatic doping in the films is twice as strong in cuprates as in nickelates. We demonstrate that the depletion of the self-doping rare-earth 5d states enhances the similarity of nickelate and cuprate Fermi surfaces in film geometry, reflecting a single hole in the Ni and Cu 3 d x 2 ??y 2 orbitals. Finally, we show that NdNiO 2 films grown on a polar NdGaO 3 (001) substrate feature a simultaneous suppression of q2DEG formation as well as Nd~ 5d self-doping.

Local spins coupled to superconductors give rise to several emerging phenomena directly linked to the competition between Cooper pair formation and magnetic exchange. These effects are generally scrutinized using a spectroscopic approach which relies on detecting the in-gap bound modes arising from Cooper pair breaking, the so-called Yu-Shiba-Rusinov (YSR) states. However, the impact of local magnetic impurities on the superconducting order parameter remains largely unexplored. Here, we use scanning Josephson spectroscopy to directly visualize the effect of magnetic perturbations on Cooper pair tunneling between superconducting electrodes at the atomic scale. By increasing the magnetic impurity orbital occupation by adding one electron at a time, we reveal the existence of a direct correlation between Josephson supercurrent suppression and YSR states. Moreover, in the metallic regime, we detect zero bias anomalies which break the existing framework based on competing Kondo and Cooper pair singlet formation mechanisms. Based on first-principle calculations, these results are rationalized in terms of unconventional spin-excitations induced by the finite magnetic anisotropy energy. Our findings have far reaching implications for phenomena that rely on the interplay between quantum spins and superconductivity.

We study the properties of two electrons with Coulomb interactions in a tight-binding model of La-based cuprate superconductors. This tight-binding model is characterized by long-range hopping obtained previously by advanced quantum chemistry computations. We show analytically and numerically that the Coulomb repulsion leads to a formation of compact pairs propagating through the whole system. The mechanism of pair formation is related to the emergence of an effective narrow energy band for Coulomb electron pairs with conserved total pair energy and momentum. The dependence of the pair formation probability on an effective filling factor is obtained with a maximum around a filling factor of 20 (or 80) percent. The comparison with the case of the nearest neighbor tight-binding model shows that the long-range hopping provides an increase of the phase space volume with high pair formation probability. We conjecture that the Coulomb electron pairs discussed here may play a role in high temperature superconductivity.

Bi-2212 is the only high-field, high-temperature superconductor (HTS) capable of reaching a critical current density J c (16 T, 4.2 K) of 6500 A?�m m ?? in the highly desirable round wire (RW) form. However, state-of-the-art Bi-2212 conductors still have a critical current density ( J c ) to depairing current density ( J d ) ratio around 20 to 30 times lower than that of state-of-the-art Nb?�Ti or REBCO. Previously, we have shown that recent improvements in Bi-2212 RW J c are due to improved connectivity associated with optimization of the heat treatment process, and most recently due to a transition to a finer and more uniform powder manufactured by Engi-Mat. One quantitative measure of connectivity may be the critical current ( I c ) distribution, since the local I c in a wire can vary along the length due to variable vortex-microstructure interactions and to factors such as filament shape variations, grain-to-grain connectivity variations and blocking secondary phase distributions. Here we compare ??0.1 m length I c distributions of Bi-2212 RWs with recent state-of-the-art very high- J c Engi-Mat powder and lower J c and older Nexans granulate powder. We do find that the I c spread for Bi-2212 wires is about twice the relative standard of high- J c Nb?�Ti well below H irr . We do not yet see any obvious contribution of the Bi-2212 anisotropy to the I c distribution and are rather encouraged that these Bi-2212 round wires show relative I c distributions not too far from high- J c Nb?�Ti wires.

We study analytically an underdamped current-biased topological Josephson junction. First, we consider a simplified model at zero temperature, where the parity of the non-local fermionic state formed by Majorana bound states (MBSs) localized on the junction is fixed, and show that a transition from insulating to conducting state in this case is governed by single-quasiparticle tunneling rather than by Cooper pair tunneling in contrast to a non-topological Josephson junction. This results in a significantly lower critical current for the transition from insulating to conducting state. We propose that, if the length of the system is finite, the transition from insulating to conducting state occurs at exponentially higher bias current due to hybridization of the states with different parities as a result of the overlap of MBSs localized on the junction and at the edges of the topological nanowire forming the junction. Finally, we discuss how the appearance of MBSs can be established experimentally by measuring the critical current for an insulating regime at different values of the applied magnetic field.

The velocities of the quasiparticles that form Cooper pairs in a superconductor are revealed by the upper critical magnetic field. Here we use this property to assess superconductivity in magic-angle twisted bilayer graphene (MATBG), which has been observed over a range of moiré band filling, twist angle, and screening environment conditions. We find that for pairing mechanisms that are unrelated to correlations within the MATBG flat bands, minima in an average Fermi velocity v ??F ??k B T c ??c /??, where ??c is the magnetic length at the critical perpendicular magnetic field, are always coincident with transition temperature maxima. Both extrema occur near flat-band van Hove singularities. Since no such association is present in MATBG experimental data, we conclude that electronic correlations that yield a band-filling-dependent pairing glue must play a crucial role in MATBG superconductivity.